B.M. Eglington a,b,, R.J. Thomas a,c, R.A. Armstrong d, F. Walraven a

Transcription

1 Precambrian Research 123 (2003) Zircon geochronology of the Oribi Gorge Suite, KwaZulu-Natal, South Africa: constraints on the timing of trans-current shearing in the Namaqua Natal Belt B.M. Eglington a,b,, R.J. Thomas a,c, R.A. Armstrong d, F. Walraven a a Council for Geoscience, Silverton, Pretoria, South Africa b Department of Geological Sciences, niversity of Saskatchewan, Saskatoon, Sask., Canada c British Geological Survey, Keyworth, Nottingham, K d PRISE, Research School of Earth Sciences, Australian National niversity, Canberra, Australia Received 6 May 2002; accepted 12 January 2003 Abstract The Oribi Gorge Suite is a voluminous suite of rapakivi granite-charnockite plutons which occur in the Mzumbe and Margate terranes of the eastern (Natal) sector of the Mesoproterozoic Namaqua Natal Belt of Southern Africa. Previous geochronological studies have provided contradictory results for the age of emplacement of the suite. New single zircon evaporation dates and detailed SHRIMP zircon analyses for three of the plutons confirm the previously observed complexity with two episodes of intrusion at 1070 and 1030 Ma. Metamorphic growth of zircon at 1030 Ma is also recorded from the Oribi Gorge pluton. The two intrusive episodes are further apart timewise ( 45 Ma) than expected for plutons belonging to a single suite. The range of ages probably reflects long-lived, post-collisional trans-current shearing in the eastern (Natal) sector of the Namaqua Natal Belt when conditions suitable for formation of similar, megacrystic, A-type granitoids were repeated. Similar granitoids from the western (Namaqua) sector of the Belt indicate that these tectonic conditions were regional in extent Elsevier Science B.V. All rights reserved. Keywords: Oribi Gorge; Natal; Granitoid; Charnockite; Zircon; Pb; Mesoproterozoic 1. Introduction Isotopic investigations of the Natal sector of the Mesoproterozoic Namaqua Natal Belt during the last decade have constrained granitoid magmatism in the Mzumbe and Margate terranes to between 1.2 and 1.0 Ga, the ages of the oldest, pre-tectonic (Mzumbe Suite tonalite) and youngest post-tectonic (Mbizana microgranite) intrusive rocks, respectively (Thomas Corresponding author. address: bruce.eglington@usask.ca (B.M. Eglington). and Eglington, 1990; Thomas et al., 1993a). The exact intrusive sequence of the voluminous and compositionally variable granitoids which were emplaced between these dates has, however, been the subject of considerable debate; in part because of equivocal field relationships and because of conflicting results from different isotope systems. The former is a result of poor outcrop and the piecemeal way in which field mapping was conducted until the extensive mapping programme of the KwaZulu-Natal office of the Council for Geoscience was completed and published as 1:250,000 scale maps and various publications /03/$ see front matter 2003 Elsevier Science B.V. All rights reserved. doi: /s (03)

2 30 B.M. Eglington et al. / Precambrian Research 123 (2003) (Thomas, 1988a,b). Geochronological studies have obtained contradictory results for individual plutons with Rb Sr systematics for the megacrystic granitoids, in particular, providing dates distinctly different to those suggested by ID-TIMS Pb zircon and zircon evaporation techniques (Eglington et al., 1986a, 1989a,b; Thomas, 1988a; Thomas et al., 1993b). The Natal sector of the Namaqua Natal Belt has been shown to comprise three discrete crustal blocks termed, from north to south, the Tugela, Mzumbe and Margate terranes (Thomas, 1994). A model for the development of the belt (Jacobs et al., 1993, 1997; Thomas et al., 1994; Jacobs and Thomas, 1994) contends that the Natal sector represents an oblique arc-continent collision orogen, with an early deformational event giving rise to NE-vergent re-folded, recumbent folds and thrusts and the development of a S- to SW-dipping pervasive metamorphic foliation. It was during this phase that the three tectonostratigraphic terranes of the sector were accreted to the southern margin of the Kaapvaal Craton. Continued northwards convergence gave rise to crustal thickening and the imposition of an oblique sinistral trans-current regime manifested in numerous WSW trending steep shear belts (Jacobs et al., 1993). The Southern two arc-related terranes (Mzumbe and Margate terranes) are dominated by granitoids which make up >80% of their exposed areas. The late tectonic Oribi Gorge Suite is the most extensive (Fig. 1). Timing of intrusion of these plutons is important, both in terms of constraining local intrusive successions and in providing a minimum date for juxtaposition of the Margate and Mzumbe terranes since the suite is present in both terranes. Generation of the Oribi Gorge Suite has also been related to the trans-current shearing event, so dating the suite also constrains the timing of this event. This paper provides single zircon evaporation dates and detailed SHRIMP zircon analyses for three plutons in order to address these issues. Petrographically and chemically similar rocks also occur in the western (Namaqua) sector of the Namaqua Natal Belt where they are classified as the Spektakel Suite. As with the Oribi Gorge Suite, these units were emplaced late in the tectonic history of the belt, associated with shear zones (Thomas et al., 1996). Fig. 1. Distribution of the Oribi Gorge Suite within the Mzumbe and Margate terranes, Natal sector of the Namaqua Natal Belt. Localities of the dated samples are indicated. 2. Geological description and regional significance of the granitoids The field geology, petrography and geochemistry of these granitoid units have been described elsewhere. Consequently, only a short summary, with references, is presented here. The Oribi Gorge Suite comprises ten plutons of very coarse-grained, porphyritic (locally rapakivi) granite and charnockite intrusive into metasediments of the Mapumulo Group (Mzumbe Terrane) and Mzimkulu Group (Margate Terrane), the Mzumbe, Humberdale, Sezela and Margate granitoid suites and the Equeefa metabasite (Eglington et al., 1986a; Talbot and Grantham, 1987;

3 B.M. Eglington et al. / Precambrian Research 123 (2003) Evans et al., 1987; Thomas, 1988b, 1994; Thomas et al., 1993b). The rapakivi phase is made up of K-feldspar phenocrysts set in a coarse-grained matrix of quartz, microcline, plagioclase, biotite, hornblende, ilmenite, apatite and zircon. The dark green porphyritic charnockite phase contains K-feldspar or plagioclase (in enderbites) with coarse-grained quartz, antiperthite, K-feldspar, hornblende, biotite, orthopyroxene ± clinopyroxene ± fayalite ± garnet, sulphide, zircon, apatite and graphite. The geochemistry is characterised by a wide range of SiO 2 values (57 75%) and tholeiitic, A-type, within-plate granite characteristics (high K 2 O, Na 2 O, FeO T,P 2 O 5, alkalis/cao, FeO T /(FeO T + MgO), Nb, Y, Zr, Zn, Ba). A previous geochronological study of the suite has shown that Pb and Pb Pb zircon dates of Ga are typically significantly older than Rb Sr whole-rock and biotite dates ( Ma) from the same samples (Thomas et al., 1993b). In addition, a Pb Pb evaporation date of 1068 ± 2 Ma, obtained from one intrusion (the Bomela locality of the Oribi Gorge pluton; Thomas et al., 1993b) is 30 Ma older than the 1037 ± 18 Ma Pb date obtained for a bulk population of zircons in samples from different localities within the same pluton in Oribi Gorge (Thomas, 1988a). The Oribi Gorge granite and charnockite pluton lies within the granulite facies Margate Terrane, as does the Port Edward enderbite pluton (Rb Sr date of 987 ± 19 Ma, Eglington et al., 1986b). The Mvenyane granite pluton (Rb Sr date of 973 ± 17 Ma, Thomas, 1988b), Fafa granite pluton (ID-TIMS small sample Pb zircon date of 1029±10 Ma, Thomas et al., 1993b) and Mgeni granite pluton (Rb Sr date of 1030 ± 20 Ma, Eglington et al., 1989a) are the only members of the suite from the Mzumbe Terrane for which dates are available. Overgrowths on zircon in the Quha Formation, sampled close to the Ntimbankulu pluton, provide dates of 1060 Ma and have been interpreted as dating intrusion of the pluton (Thomas et al., 1999). The discrepancy in Pb and Pb Pb zircon dates obtained from different plutons and exposures of plutons of the Oribi Gorge Suite raises the question whether the older zircon evaporation date obtained for the Bomela locality of the Oribi Gorge pluton is real or an artefact of the different technique. In order to test the various possibilities and to obtain a more comprehensive understanding of the geochronology of the entire suite, precise SHRIMP Pb dates were determined on the same zircon separates from three of the plutons. 3. Analytical procedures Zircon separation was performed by Elijah Nkosi and Ed Retief, using the mineral separation facilities at the CSIR, subsequently, relocated to the Council for Geoscience, Pretoria. All Rb Sr dating of Oribi Gorge Suite mineral separates and whole-rock powders referenced in this report were performed at one laboratory by BME. The zircon evaporation technique utilised follows published techniques (Kober, 1986, 1987). The specific application at the Council for Geoscience has been documented elsewhere (Grobler and Walraven, 1993). Data for each grain analysed were collected in blocks of 8 15 ratios at varying temperatures or until no lead was evaporated at a given temperature. Fully representative aliquots of the zircons separated were taken to the Research School of Earth Sciences analytical facility, Australian National niversity (AN), Canberra. They were mounted in epoxy resin together with the zircon standards SL13 and AS3 and characterised using transmitted and reflected light, back-scatter electron imaging and cathodoluminescence (CL) imaging, the latter two techniques on the SEM at the electron microscopy unit at AN and at the Council for Geoscience. Analyses were performed on either SHRIMP I or II: spots were selected on the basis of the optical and CL images obtained. Analysis on SHRIMP and subsequent data reduction were performed following previously documented techniques (Compston et al., 1984, 1992; Williams and Claesson, 1987; Claoue-Long et al., 1995). /Pb in the unknowns were normalised to a 206 Pb / 238 value of (equivalent to an age of Ma) for AS3. and Th concentrations were determined relative to those measured in the SL13 standard. Correction for common lead was made using the measured 204 Pb/ 206 Pb ratio and suitable adjustments for model common lead (Cumming and Richards, 1975). nless otherwise stated, dates are based on 207 Pb / 206 Pb ratios and an assessment of the concordance of the data.

4 32 B.M. Eglington et al. / Precambrian Research 123 (2003) Dates were calculated from SHRIMP Pb data using Geodate for Windows (Eglington and Harmer, 1999), following published techniques (Eglington and Harmer, 1993; Ludwig, 1998) and using recommended decay constants (Steiger and Jäger, 1977). ncertainties in the SHRIMP results tables and plots are at 1σ whilst weighted averages and interpreted dates for all techniques are quoted with 95% confidence uncertainties. 4. Zircon descriptions 4.1. Port Edward enderbite The zircon grains selected from the Port Edward pluton come from sample ND 175, one of the samples which was used in deriving the Rb Sr isochron date of 987 ± 19 Ma (Eglington et al., 1986b). The zircons from this sample form a homogenous assemblage of mildly elongated (l/b 3) prismatic zircons with curvilinear to blunt terminations. Most crystals are clear with well developed growth zoning evident under CL. Sulphide blebs of different sizes are present in many grains. Fig. 2e illustrates the CL characteristics of a grain from this sample Oribi Gorge pluton Zircons were selected from samples ND 215 and B1, the former from the Bomela locality near the south-east margin of the pluton and the latter from Oribi Gorge, near the centre of this 500 km 2 pluton. ND 215 was previously used to determine a preliminary evaporation zircon date of 1068 ± 2Ma(Thomas et al., 1993b) and was also one of those from which a Rb Sr isochron date of 859 ± 56 Ma was determined (Eglington et al., 1986b). Biotite from the same sample provides a 938 ± 10 Ma mineral whole-rock date (Thomas et al., 1993b). A bulk population zircon concentrate from sample B1 was previously used, together with zircons from four other samples, to obtain an isochron Pb upper intercept date of 1037 ± 18 Ma (Thomas, 1988a). These five samples also provided a whole-rock Rb Sr isochron date of 1003 ± 29 Ma (Thomas, 1988a). Zircons from sample ND 215 comprise an heterogeneous assemblage with numerous morphological varieties. Most grains are sub-spheroidal, ovoid, stubby or moderately to strongly elongated. Evidence for corrosion is common with many grains being necked. Very rare sub-rounded cores are occasionally evident with most of the population exhibiting Fig. 2. Representative cathodoluminescence (CL) images for Oribi Gorge Suite samples. (a) and (b) B1 (Oribi Gorge pluton, Gorge locality); (c) and (d) ND 215 (Oribi Gorge pluton, Bomela locality); (e) ND 175 (Port Edward pluton); (f) ND 199 (Fafa pluton). With the exception of grains (a) and (d), most grains in all the samples are dominated by fine-scale magmatic growth zoning. Grains (a) and (d) show extensive resorption. Dashed white line around (a) delineates the boundary of the grain. Dark areas within this boundary are overgrowths and replacements which were sufficiently wide to date with SHRIMP.

5 B.M. Eglington et al. / Precambrian Research 123 (2003) growth zoning. Occasional, very narrow, rims are present. Euhedral and globular opaque inclusions, stubby to elongated microlites and fluid inclusions are noted in many grains. No distinct cores were evident under CL imaging amongst the grains mounted for SHRIMP analysis. CL images of typical crystals from this sample are exhibited in Fig. 2c and d. Sample B1 contained grains with occasional distinct rims (Fig. 2a) and possible older cores evident in CL images. The majority of the population, however, exhibited clear growth zoning (Fig. 2a and b). As with sample ND 215, the grains often displayed rounded and necked shapes thought to reflect magmatic corrosion Fafa pluton This granite has previously been dated using zircons from sample ND 199 (Thomas et al., 1993b), providing a date of / 10 Ma. This sample, together with five other whole-rock powders, provided a Rb Sr isochron date of 878 ± 22 Ma (Eglington and Fig. 3. Evaporation 207 Pb/ 206 Pb results for samples from the Oribi Gorge Suite. Samples are: ND 199, Fafa pluton; ND 215, Oribi Gorge pluton, high-temperature data shown as filled histogram, low-temperature data shown as dashed curve; ND 175, Port Edward pluton.

6 34 B.M. Eglington et al. / Precambrian Research 123 (2003) Kerr, 1989). Biotite from the same sample provides a 915 ± 19 Ma mineral whole-rock date (Thomas et al., 1993b). Zircons were taken from the remaining concentrate from ND 199 for this study. Elongate crystals (l/b 7) predominate over stubby and equant grains. Both strongly zoned and nearly structureless crystals with numerous sulphide and fluid inclusions occur together. The strongly zoned crystals consist of well developed, elongated interior domains with round terminations but growth zones are mirrored in the adjoining outer domain, indicating that the interiors are not xenocrystic cores. CL images support this interpretation. The interior domains are typically strongly metamict and altered, suggesting that early crystallisation of zircon was enriched in relative to the later crystallising phase (e.g. see Fig. 2f). 5. Zircon evaporation dating results Four zircon grains from sample ND 199 of the Fafa granite were evaporated (Table 1). They gave a multimodal summed probability distribution (Fig. 3). The form of the distribution can not be related to variations in evaporation temperature, nor to variations in Th/ content (as indicated by 208 Pb/ 206 Pb). The main Table 1 Summarised results for evaporation analysis of samples ND 175, Port Edward pluton; ND 215, Oribi Gorge pluton; ND 199, Fafa pluton Grain Temperature ( C) Number of ratios 207 Pb/ 206 Pb Precision 208 Pb/ 206 Pb Precision Sample: Fafa granite ND Sample: Oribi Gorge pluton (Bomela locality) ND Sample: Port Edward enderbite ND Data for each grain analysed were collected in blocks of 8 15 ratios. The table presents weighted averages of several blocks of data in cases where these were collected at constant temperature. Data for grains 2, 3 and 4 of sample ND 175 for differing temperatures all have equivalent 207 Pb/ 206 Pb ratios, hence weighted averages are presented.

7 B.M. Eglington et al. / Precambrian Research 123 (2003) summed probability peak occurs at (equivalent to a date of 1005 Ma) with lesser peaks at (945 Ma), (905 Ma) and (1055 Ma). Six grains of sample ND 215 from the Oribi Gorge charnockite (Bomela locality) were evaporated (Table 1) and provided a multimodal summed probability distribution (Fig. 3). Omission of the lower temperature components of the distribution provides an essentially unimodal distribution with a peak at 207 Pb/ 206 Pb = , equivalent to a date of 1068 Ma. No Th/ variation is evident in the evaporation data. Four zircon grains from the Port Edward pluton (ND 175) were evaporated and the results are summarised in Table 1 and portrayed in Fig. 3. The summed probability distribution for the 207 Pb/ 206 Pb ratios is bimodal with the major peak at a 207 Pb/ 206 Pb value of (equivalent to a date of 1020 Ma) and a lesser peak at (equivalent to a date of 1060 Ma). There is no distinction between low- and high-temperature evaporation results for the grains, nor are any variations in Th/ content of the evaporated phases evident. 6. SHRIMP Pb dating results 6.1. Port Edward enderbite Twelve spots were analysed on eleven grains from sample ND 175, concentrating on areas exhibiting growth zoning revealed by CL imaging. Together, the data define a concordia date of 1025 ± 8 Ma (probability of fit = 0.43, Fig. 4, Table 2). No distinct cores were evident amongst the grains mounted or analysed. The SHRIMP result is similar to the major peak obtained from evaporation analysis (Thomas et al., 1993b). The older peak obtained in the evaporation Fig. 4. Wetherill plot of SHRIMP analyses for sample ND 175 from the Port Edward enderbite. The individual analyses (1σ error ellipses), define a concordia date and weighted average error ellipse shown in black.

8 36 B.M. Eglington et al. / Precambrian Research 123 (2003) spectrum thus presumably represents some older component present in the population but not analysed or resolvable in the limited number of spots determined with SHRIMP Oribi Gorge pluton Sixteen spots were analysed on sixteen grains from sample ND 215, again concentrating on areas exhibiting growth zoning in the CL images (Table 3). These sixteen spots define a concordia date of 1082 ± 7 Ma (probability of fit = 0.44, Fig. 5), similar to, but not statistically equivalent to the 1068 ± 5 Ma evaporation result provided earlier (Thomas et al., 1993b). Forty spot analyses were determined for seventeen grains of sample B1 (Table 4). nlike the grains mounted for sample ND 215, this sample contained discernible rims (see Fig. 2a), cores and domains with prominent growth zoning. These domains have been treated separately in the geochronological calcu- lations. Where spots analysed span two domains, as identified in subsequent SEM imaging, these analyses have been omitted from the calculations. The association of the spots with specific domains are marked accordingly in Table 4. The concordia date for the six core domains analysed is 1071±12 Ma (probability of fit = 0.53, Fig. 6) whereas the magmatic domains (omitting one rather discordant analysis, spot 10.3) define a concordia date of 1063 ± 5 Ma (probability of fit = 0.97, Fig. 6). These two sets of analyses are statistically equivalent and together, define a concordia date of 1064 ± 5Ma (probability of fit = 0.96) The seven rim spots analysed define a concordia date of 1029±8 Ma (probability of fit = 0.71, Fig. 6). The rims have higher /Th ratios than the other domains, suggesting growth under different conditions. Since the rims cut across and replace the prominent growth zoning (assumed to be of magmatic origin) and are distinctly younger, they are considered to reflect metamorphic overgrowths. Fig. 5. Wetherill plot of SHRIMP analyses for sample ND 215 from the Bomela locality, Oribi Gorge pluton. The individual analyses (1σ error ellipses), define a concordia date and weighted average error ellipse shown in black.

9 Table 2 SHRIMP Pb results for sample ND 175, Port Edward pluton Grain spot Th Th/ Pb 204 Pb/ 206 Pb f 206 (%) Radiogenic ratios Ages (in Ma) Concentration 206 Pb/ Pb ± 206 Pb/ Pb ± (%) a b Notes: (1) ncertainties given at the 1σ level. (2) f 206 denotes the percentage of 206 Pb that is common Pb. (3) For concentration, 100% denotes a concordant analysis. B.M. Eglington et al. / Precambrian Research 123 (2003)

10 Table 3 SHRIMP Pb results for sample ND 215, Oribi Gorge pluton Grain spot Th Th/ Pb 204 Pb/ 206 Pb f 206 (%) Radiogenic ratios Ages (in Ma) Concentration 206 Pb/ Pb ± 206 Pb/ Pb ± (%) Notes: (1) ncertainties given at the 1σ level. (2) f 206 denotes the percentage of 206 Pb that is common Pb. (3) Correction for common Pb made using the measured 204 Pb/ 206 Pb ratio. (4) For concentration, 100% denotes a concordant analysis. 38 B.M. Eglington et al. / Precambrian Research 123 (2003) 29 46

11 Table 4 SHRIMP Pb results for sample B1, Oribi Gorge pluton Grain spot Th Th/ Pb 204 Pb/ 206 Pb f 206 (%) Radiogenic ratios Ages (in Ma) Concentration 206 Pb/ Pb ± 206 Pb/ Pb ± (%) r r r r c c r m c c c m c r r Notes: (1) ncertainties given at the 1σ level. (2) f 206 denotes the percentage of 206 Pb that is common Pb. (3) Correction for common Pb made using the measured 204 Pb/ 206 Pb ratio. (4) For concentration, 100% denotes a concordant analysis. (5) Denotes repeat analysis on the same spot. (6) CL and other characteristics: r: dark CL, low Th/ phase; c: apparent core; m: mixed analysis. B.M. Eglington et al. / Precambrian Research 123 (2003)

12 Table 5 SHRIMP Pb results for sample ND 199, Fafa pluton Grain spot Th Th/ Pb 204 Pb/ 206 Pb f 206 (%) Radiogenic ratios Ages (in Ma) Concentrtion 206 Pb/ Pb ± 206 Pb/ Pb ± (%) B.M. Eglington et al. / Precambrian Research 123 (2003) Notes: (1) ncertainties given at the 1σ level. (2) f 206 denotes the percentage of 206 Pb that is common Pb. (3) Correction for common Pb made using the measured 204 Pb/ 206 Pb ratio. (4) For concentration, 100% denotes a concordant analysis.

13 Table 6 Summary of results for various Oribi Gorge Suite plutons Pluton Technique Date (Ma) ± 95% confidence uncertainity Interpretation Reference Mgeni Zr bulk population 1030 ± 20 Emplacement Eglington et al. (1989b) Ap bulk population 940 ± 7 Cooling Eglington et al. (1989b) Rb Sr wr 1001 ± 35 Emplacement Eglington et al. (1989b) Rb Sr bi 940 Cooling Nicolaysen and Burger (1965) Fafa Zr SHRIMP 1037 ± 10 Emplacement This study Zr ID-TIMS / 10 Emplacement Thomas et al. (1993b) Zr ID-TIMS + SHRIMP 1037 ± 7 Emplacement Thomas et al. (1993b); this study Rb Sr wr 878 ± 22 Cooling Eglington and Kerr (1989) Rb Sr bi 915 ± 19 Cooling Thomas et al. (1993b) Mvenyane Rb Sr wr 991 ± 39 Cooling Thomas (1988a) Rb Sr bi 973 ± 17 Cooling Thomas et al. (1993b) Oribi Gorge (in Oribi Gorge) Cores Zr SHRIMP 1071 ± 12 Inheritance This study Magmatic Zr SHRIMP 1063 ± 5 Emplacement This study Cores + magmatic Zr SHRIMP 1064 ± 5 Emplacement This study Rims Zr SHRIMP 1029 ± 8 Metamorphic overgrowths This study Bulk population 1038 ± 18 Mixture of inheritance, Thomas (1988a) magmatic and metamorphic growth Rb Sr wr 1003 ± 29 Thomas (1988a) Rb Sr bi 924 ± 49 Cooling Thomas et al. (1993b) Rb Sr bi 882 ± 18 Cooling Thomas et al. (1993b) Oribi Gorge (Bomela locality) Zr SHRIMP 1082 ± 7 Mixture of inheritance This study and magmatic growth Zr evaporation 1068 ± 2 Mixture of inheritance Thomas et al. (1993b) and magmatic growth Rb Sr wr 891 ± 56 Cooling Eglington et al. (1986b) Rb Sr bi 938 ± 10 Cooling Thomas et al. (1993b) Oribi Gorge (both localities) Cores + magmatic Zr SHRIMP 1070 ± 4 Emplacement This study Port Edward Zr SHRIMP 1025 ± 8 Emplacement This study Rb Sr wr 987 ± 19 Cooling Eglington et al. (1986b) Rb Sr bi 990 Cooling Nicolaysen and Burger (1965) B.M. Eglington et al. / Precambrian Research 123 (2003)

14 42 B.M. Eglington et al. / Precambrian Research 123 (2003) Fig. 6. Wetherill plot of SHRIMP analyses for core, magmatic and rim in zircons for sample B1 from Oribi Gorge near the centre of the Oribi Gorge pluton. The individual analyses (1σ error ellipses), define concordia dates with weighted average error ellipses shown in black. Data for the core and magmatic features of sample B1 are also statistically similar to the data for sample ND 215. Combining these results provides a concordia date of 1070 ± 4 Ma (probability of fit = 0.48) which is considered the best estimate for intrusion of the Oribi Gorge pluton Fafa granite Eighteen spots were analysed on fourteen grains of sample ND 199, primarily concentrating on areas with clear growth zoning evident on the CL images. Data are provided in Table 5. Regression of the data for all eighteen spots provides an upper intercept date of 1037 ± 10 Ma with a lower intercept of 22 ± 82 Ma (probability of fit = 0.97, Fig. 7). Attempts to date domains which resembled cores did not provide older dates consistent with inheritance. The date of 1037 ± 10 Ma is within error of the small sample, ID-TIMS result but is older than the dominant peak evident in the evaporation data (Thomas et al., 1993b). Regressing the combined ID-TIMS and SHRIMP data provides a date of 1032 ± 7 Ma (probability of fit = 0.95) which is considered the age of emplacement of this pluton. Results for the various Oribi Gorge Suite plutons are summarised in Table Discussion of zircon geochronology The Oribi Gorge Suite in KwaZulu/Natal comprises several late- to post-tectonic megacrystic granitoids

15 B.M. Eglington et al. / Precambrian Research 123 (2003) Fig. 7. Wetherill plot of SHRIMP analyses of domains exhibiting concentric growth zoning in zircons for sample ND 199 from the Fafa pluton. The individual analyses (1σ error ellipses), define an upper intercept date of 1037 ± 10 Ma. (Thomas, 1988a,b). The zircon data presented here for the Fafa, Port Edward and Oribi Gorge plutons have provided a range of dates from 1071 to 1025 Ma. Considering all the available data, there appear to have been two episodes of intrusion at 1070 and 1030 Ma (Fig. 8) with metamorphic growth of zircon at 1030 Ma in the case of the Oribi Gorge pluton. The range of 45 Ma ( Ma) is too large for the plutons to belong to one coeval suite, but no features have previously been evident in the zircons to identify any form of inheritance or younger overgrowth. Results other than 1025 Ma are only apparent from the Oribi Gorge pluton (and possibly also in the Ntimbankulu pluton, see below) but the variation calls into doubt the correlation of all plutons included in the suite as coeval. The Oribi Gorge pluton has been dated from two localities: bulk population Pb zircon from samples within Oribi Gorge (Thomas, 1988a; Thomas et al., 1993b) and from a locality near Bomela railway siding. Rb Sr dating from these localities provides evidence for substantial open-system behaviour (Eglington et al., 1986a; Thomas et al., 1993b), despite the presence of fayalite at the Bomela locality. Dating of core, magmatic and rim features from sample B1 demonstrates that the Oribi Gorge pluton was emplaced at 1070 Ma and was subsequently metamorphosed at 1030 Ma. Older cores within the magmatic domains have ages statistically equivalent to the magmatic domains, suggesting that the source material for these cores was only slightly older than the magmatism which produced the Oribi Gorge pluton or that zircon growth occurred in two pulses within the magma. The 1070 Ma date obtained for the Oribi Gorge pluton is similar to that inferred for the Ntimbankulu pluton (Thomas et al., 1999), based on the growth of 1060 Ma rims on older zircons in the Quha Formation close to the margin of the pluton. In contrast, the Fafa and Port

16 44 B.M. Eglington et al. / Precambrian Research 123 (2003) Fig. 8. Plot of dates obtained for the Oribi Gorge Suite relative to summed probability distributions of dates from the Namaqua Natal Belt and from late-orogenic granitoids of the Spektakel Suite in the Western (Namaqua) sector of the belt (Barton, 1983; Thomas et al., 1996; Robb et al., 1999; Eglington, 2000a,b). Edward plutons were intruded at 1030 Ma. Since all the Oribi Gorge Suite plutons are thought to have been produced as a result of melting associated with trans-current shearing late in the history of the belt, this activity must have been repeated at least twice (in this sector of the belt) over a period of 45 Ma. Approximately 1030 (Port Edward and Fafa plutons) and 1070 Ma emplacement dates are evident from Oribi Gorge Suite plutons in both the Margate and Mzumbe terranes. These terranes must therefore have been juxtaposed by at least 1070 Ma. This is consistent with Ar Ar dates which suggest that the principle foliation-defining tectonism was concluded by 1130 Ma when the Tugela Terrane was thrust onto the southern edge of the Kaapvaal Craton (Jacobs et al., 1997). Biotite whole-rock dates for the various units provide an indication of the post-intrusion metamorphic history of these rocks. In all cases, biotite model dates are about Ma, indicating that there has not been any significant (>300 C) metamorphic overprint subsequent to this time and that post-intrusion cooling must have been quite rapid. Late tectonic, porphyritic granitoid-charnockites, classified as the Spektakel Suite, are also observed 1200 km to the west in the Namaqua sector of the belt (Thomas et al., 1996; Eglington, 2000a). These granitoids were intruded at the same time as the Oribi Gorge Suite, coincident with the principle period of high-grade metamorphism, at least as preserved in the geochronological record (Fig. 8). Similar ages are also apparent for megacrystic granite from borehole intersections of basement in the intervening area (Eglington and Armstrong, 2003). The large geographic spread of these various granitoids suggests that the orogenic processes involved in their formation are regional in extent, probably associated with the escape tectonic structures late in the evolution of the belt (Jacobs et al., 1993) when localised transtensional conditions occurred along the sinistral shear zones. 8. Conclusions Timing of intrusion of the Oribi Gorge Suite plutons is important, both in terms of constraining local

17 B.M. Eglington et al. / Precambrian Research 123 (2003) intrusive successions and in providing a minimum date for juxtaposition of the Margate and Mzumbe terranes since the suite is present in both terranes. The dates from various domains of zircons from the suite range from 1071 to 1025 Ma, indicating that terrane accretion was complete by this stage. There appear to have been two episodes of intrusion at 1070 and 1030 Ma. Metamorphic growth of zircon at 1030 Ma is evident in the Oribi Gorge pluton. Considering only those dates interpreted as dating emplacement, the resulting range of 45 Ma ( Ma) is larger than expected for plutons belonging to a single coeval suite. The dating implies episodic generation during the late-tectonic trans-current shearing event, so this activity occurred over a period of at least 45 Ma. This is consistent with the assertions of Jacobs and Thomas (1994) and Jacobs et al. (1997) that the shearing event was a long-lived tectonic episode. Similar ages for late tectonic porphyritic granitoidcharnockites are also observed in the Namaqua sector of the belt, more than 1000 km further west. The wide spread in similar granitoid emplacement suggests that the processes involved relate to major regional orogenic tectonism, probably associated with the escape tectonic structures late in the evolution of the belt (Jacobs et al., 1993). Acknowledgements Jock Harmer is thanked for the many stimulating discussions as regards the development of the Natal sector of the Namaqua Natal Belt. Elijah Nkosi provided excellent assistance in crushing and mineral separation, while Dr. Ed Retief is thanked for the zircon descriptions. The Electron Microscopy nit at AN is thanked for providing access to their SEM on which most of the CL imagery was performed. Fernando Corfu and Jay Barton provided constructive reviews which substantially benefitted the final manuscript. References Barton, E.S., Reconnaissance isotopic investigations in the Namaqua Mobile Belt and implications for Proterozoic crustal evolution Namaqualand geotraverse. Spec. Publ. Geol. Soc. S. Afr. 10, Claoue-Long, J.C., Compston, W., Roberts, J., Fanning, C.M., Two Carboniferous ages: a comparison of SHRIMP zircon dating with conventional zircon ages and 40 Ar/ 39 Ar analysis, vol. 54. SEPM Special Publication, SEPM, pp Compston, W., Williams, I.S., Kirschvink, J.L., Zichao, Z., Guogan, M., Zircon Pb ages for the early Cambrian time-scale. J. Geol. Soc. Lond. 149, Compston, W., Williams, I.S., Meyer, S., Pb geochronology of zircons from lunar breccia using a sensitive high mass-resolution ion microprobe. J. Geophys. Res. 89, B525 B534. Cumming, G.L., Richards, J.R., Ore lead isotope ratios in a continuously changing earth. Earth Planetary Sci. Lett. 28, Eglington, B.M., 2000a. Geochemistry of lithologies from south-western Namaqualand. Council for Geoscience Open File Report, O, pp Eglington, B.M., 2000b. DateView desktop geochronology database. J. Afr. Earth Sci. 31, Eglington, B.M., Armstrong, R.A., Geochronological and isotopic constraints on the Mesoproterozoic Namaqua Natal Belt: evidence from deep borehole intersections in South Africa. Precambrian Res. 2003, in press. Eglington, B.M., Harmer, R.E., A review of the statistical principles of geochronometry: II. Additional concepts pertinent to radiogenic Pb studies. S. Afr. J. Geol. 96, Eglington, B.M., Harmer, R.E., GEODATE for Windows version 1: isotope regression and modeling software. Council for Geoscience Open File Report, O, pp Eglington, B.M., Kerr, A., Rb Sr and Pb Pb geochronology of Proterozoic intrusions from the Scottburgh area of Southern Natal. S. Afr. J. Geol. 92, Eglington, B.M., Harmer, R.E., Kerr, A., 1986a. Rb Sr isotope and geochemical characteristics of intrusive units from the Port Edward Port Shepstone area, Natal. Trans. Geol. Soc. S. Afr. 89, Eglington, B.M., Harmer, R.E., Kerr, A., 1986b. Petrographic, Rb Sr isotope and geochemical characteristics of intrusive granitoids from the Port Edward-Port Shepstone area, Natal. Trans. Geol. Soc. S. Afr. 89, Eglington, B.M., Harmer, R.E., Kerr, A., 1989a. Isotope and geochemical constraints on Proterozoic crustal evolution in south-eastern Africa. Precambrian Res. 45, Eglington, B.M., Harmer, R.E., Kerr, A., 1989b. Rb Sr isotopic constraints on the ages of the Mgeni and Nqwadolo granites, valley of a thousand hills, Natal. S. Afr. J. Geol. 92, Evans, M.J., Eglington, B.M., Kerr, A., Saggerson, E.P., The geology of the Proterozoic rocks around mzinto, Southern Natal, South Africa. S. Afr. J. Geol. 90, Grobler, D.F., Walraven, F., Geochronology of Gaborone granite complex extensions in the area north of Mafikeng, South Africa. Chem. Geol. 105, Jacobs, J., Thomas, R.J., Oblique collision at 1.1 Ga along the Southern margin of the Kaapvaal continent, south-east Africa. Geologische Rundschau 83, Jacobs, J., Falter, M., Thomas, R.J., Kunz, J., Je berger, E.K., Ar/ 39 Ar thermochronological constraints on the

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